This application is related to concurrently filed application titled xe2x80x9cReflective Electrophoretic Display With Stacked Color Cellsxe2x80x9d(IBM Docket ARC920010075US1).
The present invention relates to electrophoretic cells that form an electrophoretic display. In particular the invention relates to a stacked cell configuration for use in a color electrophoretic display operating in a light-transmissive mode.
An electrophoretic cell is a cell comprised of pigment particles suspended in a fluid and uses electrophoresis to switch between the following two states:
Distributed State: Particles are positioned to cover the horizontal area of the cell. This can be accomplished, for example, by dispersing the particles throughout the cell, by forcing the particles to form a layer on the horizontal surfaces of the cell, or by some combination of both.
Collected State: Particles are positioned to minimize their coverage of the horizontal area of the cell, thus allowing light to be transmitted through the cell. This can be accomplished, for example, by compacting the particles in a horizontal area that is much smaller than the horizontal area of the cell, by forcing the particles to form a layer on the vertical surfaces of the cell, or by some combination of both.
The electrophoretic cell can serve as a light valve since the distributed and collected states can be made to have different light absorbing and/or light scattering characteristics. As a result, an electrophoretic cell can be placed in the light path between a light source and a viewer and can be used to regulate the appearance of a picture element or xe2x80x9cpixelxe2x80x9d in a display. The basic operation of transmissive electrophoretic cells along with the examples of various electrode arrangements are described in IBM""s U.S. Pat. Nos. 6,144,361 and 6,184,856.
Transmissive color displays are known that use liquid crystals and crossed-polarizers to control the intensity of light through the color filters in each pixel. The use of linear polarizers limits the transmission of light through the display and, hence the backlight efficiency, brightness and power efficiency of these displays is reduced. These displays also suffer from limited viewing angle.
Liquid crystal displays commonly use a side-by-side arrangement of single color subpixels within each pixel to generate color via spatial color synthesis. The transmission efficiency of such an arrangement is limited by the fact that each subpixel only occupies a fraction of the total pixel area. By arranging the subpixels in a vertical stack, each subpixel can occupy the same lateral area as the pixel itself, and the transmission efficiency can be significantly increased. U.S. Pat. No. 5,625,474 assigned to the Sharp Corporation and IBM""s U.S. Pat. No. 5,801,796 describe embodiments of stacked cell arrangements suitable for liquid crystal materials. Since these embodiments rely on liquid crystal materials, however, they remain hindered by the aforementioned limitations of such materials. Electrophoretic displays do not suffer from these limitations and can offer improved transmission characteristics combined with extremely low power requirements.
Electrophoretic color displays have been proposed in the prior art. Japanese Patent JP 1267525 assigned to Toyota Motor Corporation describes an electrophoretic display having colored (blue and yellow) particles with different zeta potentials in a solution of red dye to give a multicolored (yellow, green and red) display. When a certain voltage is applied to the pixels, the yellow particles are pulled to the front transparent electrode and the viewer sees yellow. At a higher voltage, the blue particles are also pulled to the front electrode and the viewer sees green. When the particles are pulled off the transparent electrode, the colors of the particles are hidden by the dye solution and the viewer sees red.
U.S. Pat. No. 3,612,758 assigned to Xerox Corporation describes an electrophoretic display having pigment particles of a single color in a contrasting dye solution. In this scheme, under the influence of an electric field, the particles migrate to a front transparent electrode and the viewer sees the color of the particles. When the field is reversed, the particles migrate away from the front transparent electrode, are hidden in the dye solution, and the viewer sees the color of the dye solution.
WO 94/28202 assigned to Copytele Inc. describes a dispersion for an electrophoretic display comprised of two differently colored particles that are oppositely charged. The polarity of the voltage applied to the cell determines the polarity of the particle attracted to the front transparent electrode, and hence determines the color seen by the viewer.
In the electrophoretic color display references cited above, the use of a backlight is not suggested nor would these embodiments have contrast in a transmissive mode of operation. Transmissive electrophoretic displays based on backlit operation, however, have been proposed in the prior art.
U.S. Pat. No. 4,218,302 assigned to the U.S. Philips Corporation, describes a transmissive electrophoretic display that uses particles to either allow or frustrate the total internal reflection of light at the interface between the interior of the rear window and the suspension fluid. In the regions of this interface where no particles are present, the conditions for total internal reflection are satisfied, and light from the source is reflected back towards the source and the viewer sees no light. In the regions of this interface where the particles are present, the condition for total internal reflection is frustrated, and light from the source passes through the interface and the viewer sees light.
U.S. Pat. No. 4,648,956 assigned to the North American Philips Corporation describes several embodiments of a transmissive electrophoretic display. In these embodiments, the suspension is contained between a transparent, large-area upper electrode and a small-area lower electrode. When the absorbing particles in a selected region of the display are collected on the small-area lower electrode, they cover only a small portion of the horizontal area of the selected region. As a result, most of the light from the source passes through the selected region without being absorbed and the viewer sees light. When the absorbing particles are drawn to the upper electrode, they cover substantially the horizontal area of the selected region so that most of the light from the source is absorbed before it can reach the viewer and the viewer sees dark.
U.S. Pat. No. 5,298,833 assigned to Copytele Inc. describes a transmissive electrophoretic display based on a conductive mesh screen disposed between the backlight and the viewer. The mesh screen covers the viewing area of the display and is immersed in a suspension comprised of black particles in a clear fluid. Particles are either drawn to, or removed form a selected area of the mesh by using patterned transparent electrodes disposed above and below the mesh. When the black particles are drawn to the mesh they sit on the mesh without completely covering its holes. Light from the source is transmitted through the holes and the viewer sees light. When the particles are removed from the mesh, they are drawn to cover the selected transparent electrode. Light from the source is substantially absorbed in the area covered by the electrode and the viewer sees dark.
IBM""s U.S. Pat. No. 6,225,971 describes a transmissive electrophoretic display with improved brightness and color gamut, but which relies on spatial color synthesis. As stated above, however, the transmission efficiency of color generation via spatial color synthesis is limited by the fact that each subpixel only occupies a fraction of the total pixel area.
There is a continuing need in the art for a low-power transmissive color electrophoretic display with high brightness, wide-viewing angle, high image contrast, and large color gamut. It would be desirable, therefore, to incorporate the advantages offered by electrophoresis in a scheme that can utilize vertically stacked subpixels to maximize transmission efficiency. Electrophoretic displays that rely on hiding particles in a dye or behind a mesh are not suitable for stacking, since their contrast originates from the need to prevent light from passing through both the particles and the hiding medium. Furthermore, stacked cell structures suitable for liquid crystal materials are not appropriate for stackable electrophoretic schemes. In particular, the lateral parallel-plate electrode geometries used in both stacked and non-stacked liquid crystal displays are not capable of switching an electrophoretic suspension between its distributed and collected states. In addition, since electrophoretic suspensions can be influenced by weak electric fields, such geometries do not provide sufficient isolation of any given subpixel from the stray electric fields that originate from its neighbors.
The present invention is a transmissive electrophoretic color display. The display is intended to be viewed while illuminated from the rear by a backlight. The display is comprised of many picture elements or pixels located in lateral adjacency in a plane. Each pixel is comprised of two or more subpixels, or cells, which are vertically stacked, one directly above the other on a light-transmissive panel located at the rear or bottom of the stacks. The cells contain a light-transmissive fluid and charged pigment particles that can absorb a portion of the visible spectrum, with each cell in a stack containing particles having a color different from the colors of the particles in the other cells in the stack. The color of a pixel is determined by the portion of the visible spectrum originating from the backlight that survives the cumulative effect of traversing each cell in the stack. Each cell is comprised of light-transmissive front and rear windows, at least one non-obstructing counter electrode, and at least one non-obstructing collecting electrode. A plurality of vertical side walls extend from the rear panel and support the windows in a spaced apart relationship. The side walls are vertically aligned with one another and thus divide the display into a plurality of vertical stacks of cells, each stack forming a pixel. The side walls are vertically aligned with one another and thus divide the display into a plurality of vertical stacks of cells, each stack forming a pixel. The electrodes are controlled by solid state switches or driving elements, such as a thin film transistor or a metal-insulator-metal device, formed on the inside surface of the rear panel, with electrical connection being made vertically through holes in the windows that separate the cells in the stacks.
The amount and color of the light transmitted by each cell is controlled by the position and the color of the pigment particles within the cell. The position, in turn, is directed by the application of appropriate voltages to the collecting and counter electrodes. When the pigment particles are positioned in the path of the light that enters the cell, the particles absorb a selected portion of this light and the remaining light is transmitted through the cell. When the pigment particles are substantially removed from the path of the light entering the cell, the light can pass through the cell and emerge without significant visible change The light seen by the viewer, therefore, depends on the distribution of particles in each of the cells in the vertical stacks. Since each of the cells or subpixels in the stack occupy the same lateral area as the pixel itself, the transmission efficiency can be significantly higher than that of embodiments that rely on a side-by-side arrangement of subpixels to generate color.
A more thorough disclosure of the present invention is presented in the detailed description that follows and from the accompanying figures.